Local measurements of viscoelastic moduli of entangled actin networks using an oscillating magnetic bead micro-rheometer

A magnetically driven bead micro-rheometer for local quantitative
measurements of the viscoelastic moduli in soft macromolecular
networks such as an entangled F-actin solution is described. The
viscoelastic response of paramagnetic latex beads to external
magnetic forces is analyzed by optical particle tracking and fast
image processing. Several modes of operation are possible,
including analysis of bead motion after pulse-like or oscillatory
excitations, or after application of a constant force. The
frequency dependencies of the storage modulus, G'(omega), and the
loss modulus, G''(omega), were measured for frequencies from 10(-1)
Hz to 5 Hz. For low actin concentrations (mesh sizes epsilon >
0.1 micron) we found that both G'(omega) and G''(omega) scale with
omega 1/2. This scaling law and the absolute values of G' and G''
agree with conventional rheological measurements, demonstrating
that the magnetic bead micro-rheometer allows quantitative
measurements of the viscoelastic moduli. Local variations of the
viscoelastic moduli (and thus of the network density and mesh size)
can be probed in several ways: 1) by measurement of G' and G'' at
different sites within the network; 2) by the simultaneous analysis
of several embedded beads; and 3) by evaluation of the bead
trajectories over macroscopic distances. The latter mode yields
absolute values and local fluctuations of the apparent viscosity
eta(x) of the network.